The examination of fetal cells for early detection of fetal diseases and genetic abnormalities is carried out in connection with many pregnancies, in particular when the maternal age is high (35 years or above) or where genetic diseases are known in the family. Fetal cells may be obtained by amniocentesis, the removal of amniotic fluid from the amniotic cavity within the amniotic sac or by chorion biopsy, where biopsies are taken from the placenta, so-called invasive sampling.
Prenatal aneuploidy screening employs either traditional chromosome analysis or chromosome specific DNA probes for elucidation of numerical aberrations of the most frequently abnormal chromosomes, particular chromosomes 13, 18, 21, X and Y in the fetus.
Due to the invasiveness of the sampling methods described above and the risk of abortion, it would be advantageously to perform fetal diagnosis by a non-invasive procedure, such as for example by use of a maternal blood sample.
During pregnancy a variety of cell types of fetal origin cross the placenta and circulate within maternal peripheral blood. The feasibility of using fetal cells in the maternal circulation for diagnostic purposes has been hindered by the fact that fetal cells are present in maternal blood in only very limited numbers, reported numbers have been from one fetal cell per 105-108 nucleated maternal cells or 1-10 fetal cells per ml maternal blood. In addition most fetal cells cannot be distinguished from maternal cells on the basis of morphology alone, thus alternative methods of identification of fetal cells have been investigated.
US2007/0015171 describes a non-invasive method for isolation and detection of fetal DNA. The method enriches a maternal blood sample using antibodies that bind specifically to maternal cells and/or antibodies that bind specifically to fetal cells. The inventors suggest the use of a few specifically mentioned antibodies: HLe-1 is an antibody that recognizes an antigen present on mature human leucocytes and on very immature erythrocytes precursors, but not mature nucleated erythrocytes. Thus, it is suggested that this antibody can be used to recognize maternal leucocytes, but not fetal nucleated erythrocytes. Anti-monocyte antibody (M3) and anti-lymphocyte antibody (L4) are also suggested for removing maternal cells from a sample. Finally, the authors suggest using a monoclonal antibody, which recognizes the transferrin receptor (TfR) on fetal cells. DNA from isolated fetal cells is subsequently made available for detection and diagnosis.
WO2008/132753 describes a method of identifying a trophoblast by detecting in cells of a biological sample expression of a trophoblast marker selected from the group consisting of an annexin IV, a cytokeratin-7, a cytokeratin 8 and a cytokeratin-19. A trophoblast is referred to as an epithelial cell which is derived from the placenta of a mammalian embryo or fetus; a trophopblast typically contacts the uterine wall. Three types of trophoblasts are mentioned, the villous cytotrophoblast, the syncytiotrophoblast and the extravillous trophoblast. Importantly, the inventors used monoclonal antibodies against Vimentin to estimate the extent of fibroblast contamination of trophoblasts isolated from first trimester placentas. Thus, the trophoblasts isolated by these inventors do not comprise Vimentin.
Gussin et al., 2004, hypothesized that fetal cells in maternal blood that do not respond to hematopoietic culture conditions represent endothelial cells. They investigated whether endothelial progenitor cells of fetal origin may be selected from maternal blood on the basis of their expression of CD133 or CD105 and expanded in culture. The authors concluded that CD133+ and CD105+ cells isolated from maternal blood can be expanded in vitro under endothelial conditions. These cells appear to be of maternal, rather than fetal, origin.
Thus, there remains a need for improved methods of isolating fetal cells from maternal blood samples such as to facilitate pre-natal detection and diagnosis.